1. Field of the Invention
The present invention relates to a strained silicon substrate and the forming method thereof, more particularly, to a strained silicon substrate adapted for high-speed electronic and optical elements and the forming method thereof.
2. Description of Related Art
A strained silicon material is functioned as a substrate to various high-speed electronic elements or optoelectronic elements due to its higher electron mobility, i.e., higher carrier mobility. A strained silicon material is also applied as growing buffer layers for bonding III-V based semiconductors and IV based semiconductors, so as to integrate III-V elements (i.e. elements of group III or group V of periodic table) and IV elements (i.e. elements of group IV of periodic table) or to grow III-V elements on a silicon substrate. Since a strained silicon substrate can be integrated to grow III-V elements and IV elements to form semiconductor elements on a strained silicon-germanium epitaxy layer, and a strained silicon substrate generally replaces a silicon substrate as a substrate for growing high-speed electronics, it is commonly referred to a virtual-substrate.
Typically, a virtual-substrate is generally formed by forming a strained silicon layer over a silicon-germanium epitaxy layer on a silicon epitaxy layer. FIG. 1 is a view of a structure of the virtual-substrate. As shown in FIG. 1, the structure includes a Si substrate, a Si buffer 101, a graded silicon-germanium epitaxy layer 102, a silicon-germanium epitaxy layer 103 and a strained silicon layer 104 sequentially.
Generally, conventional silicon-germanium epitaxy layers are grown on a silicon substrate through component graded growing. The stress between silicon-germanium epitaxy layer and silicon layer is reduced by such a relaxed mechanism of component graded silicon-germanium epitaxy layer. However, when better-relaxed effect is needed, the growing time of silicon-germanium growth is too long and the thickness of a relaxed silicon-germanium epitaxy layer is too high. In addition, the alignment of pattern formation through lithography is difficult, too. Hence, no advantage in mass-production can be taken. Furthermore, a silicon-germanium epitaxy layer formed by such component-graded growth has high surface roughness, threading dislocation density and defective density, so that operation ability of electronic elements (or optoelectronic elements) to be grown completely is relatively weakened.
As cited, a heterogeneous graded epitaxy layer of thick silicon-germanium with low defective density grown by high temperature is disclosed in U.S. Pat. No. 5,221,413, which does not improve prior high thickness and long growth time as well as mass-production. In addition, the cited high roughness is improved by selecting special substrate materials (U.S. Pat. No. 6,033,803). However, mass-production of high-speed electronic elements or optoelectronic elements by improving high thickness, high roughness and long growth time at the same time does not present in current processes or products.
Therefore, it is desirable to provide an improved method to mitigate and/or obviate the aforementioned problems.